1. Field of the Invention
This invention deals with the art of desulfurizing molten metal such as iron for use in casting.
2. Prior Art
In the manufacture of cast iron, pig iron is melted usually in a cupola with coke. The molten iron in the cupola contains a considerable amount of sulfur because of the use of coke as a fuel for the cupola. Spheroidal graphite cast iron has now been extensively used because of its high quality. A spheroid-promoting agent such as silicon is added to the molten iron in the cupola to form the spheroidal graphite cast iron. However, the sulfur content of the molten iron prevents the graphite in the resultant cast iron from becoming spheroidal to a desired degree. Therefore, it is a common practice in the art to desulfurize the molten iron before the spheroid-promoting agent is applied to the molten iron. The desulfurization is carried out by applying a pulverized desulfurizing agent to the molten iron and then agitating this molten iron. This agitation is necessary for promoting the reaction of the desulfurizing agent with the sulfur in the molten iron. Calcium carbide, calcium oxide or the like is used as the desulfurizing agent. In the case where calcium carbide is used as the desulfurizing agent, the following reaction is obtained at the time of the desulfurization: EQU CaC.sub.2 +S.fwdarw.CaS+2C
CaS constitutes a slag and floats on the top of the molten iron. This slag is gasified and dissipated into the ambient atmosphere when the molten iron is at elevated temperatures. Since the desulfurizing agent has a relatively low specific gravity, it is necessary to agitate the molten metal to promote the above-mentioned reaction. One such desulfurizing method is known as a porous plug method. In this method, a porous plug is mounted on the bottom of a vessel for holding the molten iron. The desulfurizing agent is applied to the molten iron in the vessel, and inert gas such as nitrogen gas is introduced into the vessel through the porous plug to produce bubbles which move upwardly to agitate the molten iron. Another conventional desulfurizing method is known as a gas injection method. In this method, a pipe made of a refractory material is introduced into the molten iron in a vessel. Powder of carbide carried by nitrogen gas is fed through the refractory pipe to the molten iron in the vessel, thereby causing the agitation of the molten iron to effect the desulfurization thereof. These two desulfurizing methods have been found disadvantageous in that the molten iron in the vessel is subjected to a relatively large temperature drop because of the agitation of the molten iron. Therefore, it has heretofore been necessary to provide a separate heating means for heating the desulfurized molten iron to compensate for the temperature drop thereof. This is undesirable from an economical point of view. For example, the temperature of the molten iron discharged from the cupola is 1550.degree. C., and the temperature of the molten iron in the desulfurizing apparatus of the porous plug type is 1450.degree. C. And, the temperature of the desulfurized molten iron discharged from the desulfurizing apparatus is 1400.degree. C. The desulfurized molten iron is transferred to molds to form castings. The temperature of the molten iron required for casting products of relatively small size or thickness is 1400.degree. to 1430.degree. C. The molten iron discharged from the desulfurizing apparatus is further subjected to a temperature drop during a time period when it is transferred to the molds. Thus, the temperature of the molten iron discharged from the desulfurizing apparatus is not practical. For this reason, the above-mentioned heating means such as an induction furnace is required, which increases the installation cost.